The invention relates to a plasmon resonance sensor for biological, biochemical or chemical tests, with an optically transparent body, in particular a glass prism, a reflective metal layer or semiconductor layer which is applied to one face of the body and has a surface sensitive to molecules to be detected in a sample, which forms a measurement cell in conjunction with a cuvette, a monochromatic light source, in particular a laser diode, for emitting a divergent light pencil or beam path through the optically transparent body onto the inner face of the layer, and a detector which is assigned to the emerging beam path reflected by the layer and registers as a function of time the light emergence angle, which changes as a result of molecule buildup on the sensitive surface, at which an emerging light intensity minimum occurs owing to resonance.
Such a plasmon resonance sensor, with a glass prism, a thin gold layer of from 40 to 70 nm and a light source in the form of a laser diode, is known from U.S. Pat. No. 4,844,613.
The phenomenon of surface plasmon resonance (SPR) involves collective excitation of the electrons at the surface of a layer which has free electrons. The resonant frequency of the surface plasmons is very sensitive to the refractive index of the medium which is adjacent to the sensitive surface. This can be used in order to analyze thin layers (refractive index or layer thickness). Especially in biosensor technology, this effect is used in order to study the buildup kinetics of biomolecules on a functionalized metal surface. To that end, the resonance condition of the surface plasmons is detected in a temporally resolved fashion. The surface plasmons of the thin metal layer are excited by light which shines through the glass onto the metal layer at a particular angle or in a particular angle range. The resonance condition is then satisfied for a particular combination of wavelength and angle of incidence. Under this resonance condition, the intensity of the light reflected from the metal layer is reduced significantly owing to the generation of surface plasmons. In order to find the resonance condition, it is possible to scan either the angle of incidence (at constant wavelength) or the wavelength (at constant angle of incidence), and to detect the intensity of the reflected light.
In the plasmon resonance sensor described in the introduction, it is necessary to operate with a fixed wavelength and to determine the angle of incidence for which the resonance condition is satisfied. In this case, a laser diode which emits an elliptical beam cone is used. The aperture angles are typically 22xc2x0 in one dimension and 9xc2x0 in the other dimensionxe2x80x94in each case at half of the intensity maximum (FWHM). This beam divergence is used in order, without any beam shaping optics and without any change in the alignment of the light source in relation to the reflective layer, to illuminate the latter with light at different angles of incidence within an angle range which is compatible with achieving the resonance condition. Accordingly, an elongate detector arrangement is provided, which picks up the divergent emerging beam path over its full extent in the light incidence plane, and can hence determine the angle of incidence for which the resonance condition is satisfied at the time of measurement.
Since it makes do without beam shaping optics and devices for changing the angle of incidence of the light, this known plasmon resonance sensor has a comparatively simple design and is therefore economical to manufacture. However, light beams with a different angle of incidence strike different points on the reflective metal layer, so that the uniformity of the latter must be subject to stringent requirements in order to prevent falsification of the measurement results. Nevertheless, it is possible to apply metal layers which are sufficiently uniform in this sense.
The essential disadvantage of the known design is therefore the fact that the plasmon resonance sensor, which is equipped with a single measurement cell, has a low performance in terms of the number of tests which can be carried out, and it does not permit any simultaneous reference measurements in order to eliminate the effect, for example, of the reflective metal layer becoming heated. Indeed, it is precisely because of the strong temperature dependence of the refractive index of liquids, and since the samples to be studied are normally studied when they are dissolved in liquid, that reference measurements are particularly useful. It should also be noted in this context that, owing to the divergent beam path, any additional measurement cells need to be arranged at a large distance from one another, so as to avoid overlap between different beam cones and therefore falsifications. Such a spacing, however, would conflict with the desired compact configuration and also significantly increase the costs for correspondingly large components.
It is already known from EP 305 109 B1, in the case of a comparable plasmon resonance sensor for carrying out biological tests, to operate with a parallel-radiating light source and to generate therefrom, by means of optics, a convergent beam fan with all the required angles of incidence, with optics that restore parallel alignment of the beam path before it strikes the detector also being provided in the divergent emerging beam path. In this plasmon resonance sensor, the light is focused onto one point on the metal layer, so that the effect of nonuniformities of the metal layer is substantially eliminated. The drawback of this, however, is that it is necessary to tolerate increased heating of the metal layer and results which are thereby falsified. Another disadvantage of the known plasmon resonance sensor involves the comparatively expensive beam shaping optics. Moreover, additional measurement cells to increase performance and for reference measurements would also require additional corresponding beam shaping optics, and they hence make the plasmon resonance sensor significantly more expensive.
It is therefore an object of the invention to provide a plasmon resonance sensor which permits high test performance with simultaneously error-free results, while having a compact and inexpensive design.
On the basis of the plasmon resonance sensor described in the introduction, this object is achieved according to the invention by the fact that collimation optics, which collimate the incident beam path perpendicularly to the incidence plane but still leave it divergent in the incidence plane, are arranged between the light source and the optically transparent body.
Expedient refinements and developments of the invention are given in the dependent claims.
The plasmon resonance sensor according to the invention makes do with simple beam shaping optics in the form of a cylindrical lens, and it hence requires only minor equipment outlay. Although the original beam divergence, for example of a laser diode, is used in order to cover the full appropriate range of angles of incidence, the intentional parallel alignment of the beam path in a direction perpendicular to the incidence plane provides a narrow beam path in this direction, which permits compact side-by-side arrangement of a plurality of equivalent plasmon resonance sensors, and therefore of a plurality of measurement cells, and this leads to a high-performance device with the opportunity for advantageous reference measurements.
Expediently, however, this result is achieved not by sequential arrangement of a plurality of complete plasmon resonance sensors, but instead by arranging two or more measurement cells for different samples, which are aligned in a row perpendicularly to the incidence plane, on a common optically transparent body or prism, with each measurement cell being assigned its own detector. Such a design with a common optically transparent body or prism, and optionally only one light source and single collimation optics, leads to a particularly low cost outlay in terms of performance.